The invention concerns a pressing cushion for use in laminating presses, having a textile support with threads, at least some of which constitute thermally conductive threads that bring about, either directly or by contact with other thermally conductive threads, a thermal transfer from one outer side to the other outer side of the pressing cushion, the support comprising a cushion layer made of a flexible rubber material.
The manufacture of layered materials, for example particle boards equipped with decorative laminates, is accomplished in laminating presses that can be configured as low- or high-pressure multi platen presses or short-cycle presses. Pressing cushions, whose purpose is to transfer pressure uniformly onto the pressed material over its entire surface, are utilized in this context. The pressing cushions must be able to withstand high pressures as well as the temperatures present in such pressing cushions, and they must be capable of transferring the heat proceeding from the press plates quickly and without major losses onto the pressed material, i.e. they must have high thermal conductivity in that direction.
Pressing cushions are generally textile products, and there are numerous different embodiments. The pressing cushion of the species as defined in DE-B-23 19 593 comprises a plain-weave metal fabric that is completely embedded in a cushion layer, made of a silicone elastomer, that constitutes a matrix. Additives, for example made of powdered copper and/or aluminum and/or aluminum bronze powder and/or graphite and/or ferrosilicon, can be mixed into the silicone elastomer in order to increase the thermal conductivity of the cushion layer.
This type of pressing cushion has proven successful because the cushion layer made of silicone elastomer is resistant to the temperatures present in laminating presses, which can exceed 200xc2x0 C.; and because it has good long-term spring-back properties so that permanent impressions do not occur (this is also called xe2x80x9cwindowingxe2x80x9d because of the shape of the panels being pressed). A further advantage is the fact that the threads of the support are physically immobilized by the cushion layer, and the pressing cushion thus has good dimensional stability.
In order to increase productivity, efforts are being directed toward shorter and shorter cycle times in laminating presses. This requires rapid thermal transfer from the heating plates to the pressed material, i.e. the pressing cushions must have high thermal conductivity in the thickness direction.
The known pressing cushion does not satisfactorily meet these criteria despite the presence of the metal woven fabric and the additives that promote thermal conductivity. It is thus the object of the invention to configure a pressing cushion of the kind cited initially in such a way that it is also suitable for laminating presses with short cycle times, i.e. in particular, guarantees rapid thermal transfer from the heating plates to the pressed material.
According to the present invention, this object is achieved by a pressing cushion in which the thickness of the cushion layer is less than that of the support, and the cushion layer is embedded into the support in such a way that thermally conductive threads protrude beyond the cushion layer on both outer sides of the pressing cushion. The fundamental idea of the invention is thus to allow the thermally conductive threads to protrude beyond the cushion layer at least partially on both outer sides of the pressing cushion, i.e. on its flat sides, thus creating a direct (and, under press pressure, planar) contact with the heating plates on the one hand and the pressing sheets on the other hand. It has been found that thermal transfer in the thickness direction can thereby be substantially accelerated. By way of the number and thickness of the thermally conductive threads, and in particular the contact areas, the thermal conductivity in the thickness direction can be optimally adapted in accordance with particular requirements. It has been found, surprisingly, that despite the reduction in the thickness of the cushion layer, the cushioning behavior has not changed disadvantageously but in fact has improved. The ability of the pressing cushion to act in a pressure-equalizing fashion, and thus to transfer the press pressure uniformly over the surface, has not been impaired. No xe2x80x9cwindowingxe2x80x9d was observed.
Since the cushion layer will as a rule be thinner than in the case of the pressing cushion described in DE-B-23 19 593, a material saving is achieved with the pressing cushion according to the present invention, and it is not inconsiderably lighter. The pressing cushion is thus more economical to manufacture and transport. Its handling is also simpler.
To allow the pressing cushion to be used even at high temperatures, the flexible rubber or elastomer material should be resistant up to at least 200xc2x0 C., better still 240xc2x0 C. Eligible flexible rubber materials are preferably those that retain their elasticity properties over the long term at the pressures and temperatures that exist in laminating pressures. Synthetic elastomers, for example silicone elastomers, fluoroelastomers, and/or fluorosilicone elastomers, are especially suitable therefor. The flexible rubber material should preferably have a hardness in the range from 60 to 85 Shore A.
In order to obtain the best possible cushioning capability, the cushion layer should, in a manner known per se, be embedded in gap-filling fashion into the support, preferably centeredly, i.e. symmetrically. The possibility of an asymmetrical arrangement also exists, however.
Here as well, as known from the existing art, the possibility exists of incorporating filler materials into the cushion layer for various purposes. In order to reduce the weight of the pressing cushion, filler materials whose specific weight is less than that of the flexible rubber material can be distributed in the cushion layer. Gas bubbles (so that the cushion layer has a foamed character), powders, fibers, and/or microspheres are suitable for this. Instead of or in combination with these, it is also possible to incorporate thermally conductive particles, whose specific thermal conductivity is greater than that of the cushion layer, into the cushion layer. Powders or fibers made of metals or carbon are suitable for this purpose. The proportion of optionally present filler materials and optionally present thermally conductive particles should amount to 10 to 60 wt % of the weight of the cushion layer.
The cushion layer itself should constitute 15 to 40 wt %, preferably 20 to 35 wt %, of the weight of the entire pressing cushion. By way of differing weight proportions, the cushioning capability of the pressing cushion can be adapted to the particular requirements. The thickness of the pressing cushion should preferably be in the range from 0.75 to 3 mm.
The thermally conductive threads need not go back and forth between the outer sides of the cushion, although such an embodiment is preferred. Good thermal conductivity is in fact also achieved if a system of thermally conductive threads is present on both sides of the pressing cushion and the thermally conductive threads protrude there, and if said thermally conductive threads have a connection to one another in the cushion layer, for example are interwoven with one another.
As regards the number of contact points, it is proposed according to the present invention to provide 10 to 40 contact points per square centimeter. Altogether, the contact points of the thermally conductive threads on the outer sides of the pressing cushion should occupy 15 to 30% of each respective surface. These values guarantee very good thermal conductivity in the thickness direction.
The thermally conductive threads can be made of metals, in particular of highly thermally conductive metals such as, for example, copper, brass, aluminum, silver, or even an alloy thereof. Thermally conductive threads made of carbon and/or a combination of metal and carbon are also eligible. By way of the selection of the material for the thermally conductive threads, and their quantity, thickness, and arrangement, the thermal conductivity of the pressing cushion in the thickness direction can be optimally adapted to the particular requirements.
As a special instance, the thermally conductive threads can comprise a core thread that has at least one wire or ribbon wound around it or around which are spun wires or ribbons, which are made of a material whose specific thermal conductivity is greater than that of the core thread. Thermally conductive threads of this kind are evident, for example, from DE-U-297 21 494.
As in the case of the pressing cushion defined in DE-B-23 19 593, the support can be configured to consist entirely of thermally conductive threads, i.e. for example of a pure metal woven fabric. As an alternative to this, the possibility exists of providing further threads in addition to the thermally conductive threads in order to impart desired properties to the pressing cushion. The further threads can constitute plastic threads that have a temperature resistance of at least 200xc2x0 C., preferably 240xc2x0 C. Aromatic polyamides (aramids) are eligible in particular for this purpose.
With regard to the configuration of the threads (both the thermally conductive threads and the further threads), essentially no limitations are imposed. The threads can thus be configured as monofilaments, multifilaments, braided threads, fiber yarns, strands, cables, ribbons, twisted yarns, raveled yams, and/or wrapped yarns.
Provision is further made according to the invention for the support to comprise electrically conductive threads that bring about, either directly or by contact with other electrically conductive threads, an electrical conductivity between the outer sides of the pressing cushion. The static electricity which forms in laminating presses can be dissipated through electrically conductive threads of this kind. Since the materials for the thermally conductive threads usually are also electrically conductive, the electrically conductive threads can at least partially also constitute thermally conductive threads, so that the mere presence of the thermally conductive threads and their protrusion beyond the cushion layer also ensures dissipation of the static electricity.
The support is preferably configured as a continuous planar textile structure, the embodiment as a woven fabric being principally eligible. Other textile thread structures are also eligible, however, for example knitted fabrics, woven thread fabrics, or woven or knitted spacer fabrics. If the support is embodied as a woven fabric, it has proven advantageous if the weft threads extend substantially in a straight line and the warp threads form externally protruding bends, and if the latter at least partially constitute the thermally conductive threads. The height of the bends of the warp threads can be adjusted by way of the diameter of the warp and weft threads. For example, an increase in the bend height can be achieved by enlarging the warp thread diameter and reducing the weft thread diameter. It is understood that the thermal conductivity can be adjusted by way of the type of fabric weave and the thread thickness. Multiple-ply fabrics can also be used in this context.
In a further embodiment of the invention, provision is made for threads which comprise recesses that were created by the removal of thread-like components to be present, and for the material of the cushion layer to fill up the recesses. A better positive interlock between the cushion layer and threads is thereby achieved. Manufacture can be accomplished in such a way that first the support is manufactured and then the thread-like components are dissolved out by means of a solvent that dissolves them or by being burned out; and only then is the cushion layer introduced, for example by means of a ductor process. The recesses can also be created by removal of a core thread. Eligible materials for the thread-shaped components are cotton, viscose staple fiber, polyvinyl alcohol, polyethylene, polypropylene, polyamide, polyacrylonitrile, polyester, or other readily soluble or low-melting substances.
Lastly, the invention provides for the pressing cushion to have passages that are open between the outer sides of the pressing cushion, so that the pressing cushion is permeable in the thickness direction. The passages can be constituted, for example, by dissolved-out threads or thread components and/or by pores in the cushion layer.